The present invention is directed to tissue-type prosthetic heart valves and in particular to stents used in the fabrication of such valves.
Prosthetic heart valves are used to replace damaged or diseased heart valves. In vertebrate animals, the heart is a hollow muscular organ having four pumping chambers: the left and right atria and the left and right ventricles, each provided with its own one-way valve. The natural heart valves are identified as the aortic, mitral (or bicuspid), tricuspid and pulmonary valves. Prosthetic heart valves can be used to replace any of these naturally occurring valves. Two primary types of heart valve replacements or prostheses are known. One is a mechanical-type heart valve that uses a pivoting mechanical closure to provide unidirectional blood flow. The other is a tissue-type or xe2x80x9cbioprostheticxe2x80x9d valve which is constructed with natural-tissue valve leaflets which function much like a natural human heart valve, imitating the natural action of the flexible heart valve leaflets which seal against each other or coapt between adjacent tissue junctions known as commissures. Each type of prosthetic valve has its own attendant advantages and drawbacks.
Operating much like a rigid mechanical check valve, mechanical heart valves are robust and long lived but require that valve implant patients utilize blood thinners for the rest of their lives to prevent clotting. They also generate a clicking noise when the mechanical closure seats against the associated valve structure at each beat of the heart. In contrast, tissue-type valve leaflets are flexible, silent, and do not require the use of blood thinners. However, naturally occurring processes within the human body may attack and stiffen or xe2x80x9ccalcifyxe2x80x9d the tissue leaflets of the valve over time, particularly at high-stress areas of the valve such as at the commissure junctions between the valve leaflets and at the peripheral leaflet attachment points or xe2x80x9ccuspsxe2x80x9d at the outer edge of each leaflet. Further, the valves are subject to stresses from constant mechanical operation within the body. Accordingly, the valves wear out over time and need to be replaced. Tissue-type heart valves are also considerably more difficult and time consuming to manufacture.
Though both mechanical-type and tissue-type heart valves must be manufactured to exacting standards and tolerances in order to function for years within the dynamic environment of a living patient""s heart, mechanical-type replacement valves can be mass produced by utilizing mechanized processes and standardized parts. In contrast, highly trained and skilled assembly workers make tissue-type prosthetic valves by hand. Typically, tissue-type prosthetic valves are constructed by sewing two or three flexible natural tissue-leaflets to a generally circular supporting wire frame or stent. The wire frame or stent is constructed to provide a dimensionally stable support structure for the valve leaflets which imparts a certain degree of controlled flexibility to reduce stress on the leaflet tissue during valve closure. A biocompatible cloth covering on the wire frame or stent provides sewing attachment points for the leaflet commissures and cusps. Similarly, a cloth covered suture ring can be attached to the wire frame or stent to provide an attachment site for sewing the valve structure in position within the patient""s heart during a surgical valve replacement procedure.
With over fifteen years of clinical experience supporting their utilization, tissue-type prosthetic heart valves have proven to be an unqualified success. Recently their use has been proposed in conjunction with mechanical artificial hearts and mechanical left ventricular assist devices (LVADs) in order to reduce damage to blood cells and the associated risk of clotting without using blood thinners. Accordingly, a need is developing for a tissue-type prosthetic heart valve that can be adapted for use in conjunction with such mechanical pumping systems. This developing need for adaptability has highlighted one of the drawbacks associated with tissue-type valvesxe2x80x94namely, the time consuming and laborious hand-made assembly process. In order to provide consistent, high-quality tissue-type heart valves having stable, functional valve leaflets, highly skilled and highly experienced assembly personnel must meticulously wrap and sew each leaflet, and valve component into an approved, dimensionally appropriate valve assembly. Because of variations in tissue thickness, compliance and stitching, each completed valve assembly must be fine tuned using additional hand-crafted techniques to ensure proper coaptation and functional longevity of the valve leaflets. As a result, new challenges are being placed upon the manufacturers of tissue-type prosthetic valves in order to meet the increasing demand and the increasing range of uses for these invaluable devices.
Accordingly, consistent with the developing practice of the medical profession, there is a continuing need for improved tissue-type prosthetic heart valves which incorporate the lessons learned in clinical experience, particularly the reduction of stress on the valve leaflets while maintaining desirable structural and functional features. Additionally, there is a growing need for improved tissue-type prosthetic heart valves which can be adapted for use in a variety of positions within the natural heart or in mechanical pumps, such as artificial hearts or ventricular assist devices, as well as alternative locations in the circulatory system. Further, in order to address growing demand for these devices, there is a need for tissue-type heart valves that are simpler and easier to manufacture in a more consistent manner than are existing valves.
Directed to achieving the foregoing objective and to remedying the problems in the prior art, disclosed herein are novel tissue heart valve constructions and components thereof, and simplified methods of fabricating the same. The improved tissue heart valves of the present invention are fabricated to include standardized leaflet structure subassemblies that can be modified readily to adapt to different intended applications. Of equal importance, the leaflet structure subassemblies uniformly distribute tensile loads along the entire peripheral leaflet cusp, reducing stress points and significantly improving the long-term functionality of the valve assembly. As an added benefit of the present invention, the stability and adaptability of the tissue valve subassembly is achieved through simplified manufacturing processes utilizing fewer steps and subassemblies. This manufacturing protocol can be incorporated into branched, adaptable manufacturing techniques for the production of tissue heart valves having a variety of end uses. Further, these improved construction techniques expedite the overall manufacturing process and improve the consistency of the tissue valves so produced while simultaneously reducing the need for post-assembly fine tuning and quality-control procedures.
According to one aspect of the present invention, a tissue-type heart valve includes a dimensionally stable, pre-aligned tissue leaflet subassembly, a generally circular wireform, and a generally circular support stent. The wireform has a bottom surface dimensioned to receive the pre-aligned tissue leaflet subassembly in fixed, mating engagement. The support stent has an upper surface dimensioned to seat and fix in meeting engagement with the pre-aligned tissue leaflet subassembly which is fixedly disposed in mating engagement with the bottom surface of the wireform.
Pursuant to this construction, an exemplary tissue valve includes a plurality of tissue leaflets that are templated and attached together at their tips to form a dimensionally stable and dimensionally consistent coapting leaflet subassembly. Then, in what is essentially a single process, each of the leaflets of the subassembly is aligned with and individually sewn to a cloth-covered wireform, from the tip of one wireform commissure uniformly, around the leaflet cusp perimeter, to the tip of an adjacent wireform commissure. As a result, the sewed sutures act like similarly aligned staples, all of which equally take the loading force acting along the entire cusp of each of the pre-aligned, coapting leaflets. The resulting tissue-wireform structural assembly thereby formed reduces stress and potential fatigue at the leaflet suture interface by distributing stress evenly over the entire leaflet cusp from commissure to commissure. This improved, dimensionally stable, reduced-stress assembly is operatively attached to the top of a previously prepared cloth-covered stent to clamp the tissue leaflet cusps on a load-distributing cloth seat formed by the top of the cloth-covered stent without distorting the leaflets or disturbing their relative alignment and the resultant coaptation of their mating edges.
The stent is secured to the assembly with the commissures of the stent extending up into the corresponding commissures of the leaflet, wireform assembly. The stent itself can be formed of an inner polyester film support secured to a surgically acceptable metal ring such as an Elgiloy(trademark) metal stiffener having a cloth cover cut, folded and sewn around the support and stiffener combination. Alternatively, instead of having an Elgiloy outer band and a laminated polyester film support, the two stent layers can both be polyester layers or a single piece stent having appropriately flexible commissure posts. Either stent construction provides support and dimensional stability for the valve structure extending from commissure to commissure and being evenly distributed around each leaflet. This assembly methodology allows the evenly sutured tissue of the leaflet cusps to be sandwiched between the wireform and the stent and to thereby further distribute the loading forces more evenly around the attachment site. Because the tissue leaflets experience lower, more evenly distributed stresses during operation, they are less likely to experience distortion in use. Thus, a more stable, long lived, functional closure or coaptation of the leaflets is provided by this even distribution of attachment forces.
A number of additional advantages result from the present invention and the stent construction utilized therein. For example, for each key area of the stent, the flexibility can be optimized or customized. If desired, the coapting tissue leaflet commissures can be made more or less flexible to allow for more or less deflection to relieve stresses on the tissue at closing or to fine tune the operation of the valve. Similarly, the base radial stiffness of the overall valve structure can be increased or decreased to preserve the roundness and shape of the valve.
Unlike a rigid mechanical valve, the stent does not act as a rigid heart valve structure but as a radially stable, yet axially flexible support. A rigid structure is unnecessary by utilizing the teachings of the present invention because the valve leaflets are dimensionally pre-aligned along their mutually coapting mating or sealing edges prior to being directly attached to the base of the cloth-covered wireform. As a result, the entire sealing aspect of the valve can be aligned in three dimensions at once without the variability previously experienced in the construction of prior art tissue-type valves. In addition to eliminating the need for post-assembly adjustment, this pre-alignment provides for consistency and simplicity in the manufacture of the valve structure. Further, the wire form functions as a template for suturing the leaflet cusps to the valve subassembly with uniform stitching from commissure tip to commissure tip. This produces a dimensionally consistent structure that can interface with the stent in a previously unobtainable uniform manner. The consistent dimensional integrity of the leaflet wireform subassembly enables the stent to function as a stress relieving support clamp which further secures the leaflet cusps in the valve structure to provide an added degree of stability and stress distribution. If desired, providing the top of the stent with a single or double fold of covering cloth provides the stent lip with a deformable cloth seat that assists in the distribution of load around the leaflet cusps and simplifies sewing the stent to the tissue leaflet wireform subassembly. Those skilled in the art will appreciate that attaching the stent to the tissue leaflet wireform functions to stabilize the projecting commissure posts of the valve subassembly without stiffening their desirable axial flexibility. This novel construction technique eliminates the need for separate commissure posts at the tissue leaflet commissures and also eliminates multiple tissue and cloth layers at the wireform commissure posts which adds to uniformity and consistency in valve production and eliminates assembly steps. As a result, valve manufacture is not only improved, but also simplified and expedited as well.
The stent also functions as an adaptable structural interface, allowing the tissue-wireform-stent structural subassembly to be attached to a variety of additional structures dependent upon intended valve placement and operating environments. For example, with the supporting stent secured to the tissue-wireform structural assembly, the resulting valve assembly can be attached to, for example, a suture ring, a flange or a conduit depending on the desired valve application. To form a conduit valve, the suture ring can be attached directly to the inflow or base of the stent to enable the implanting surgeon to sew the valve in place within the heart. Alternatively, when the valve is to be used for artificial hearts or for left ventricular assist devices (LVADs), a more rigid flange can be attached to the stent inflow to function as a mechanical mount. In some circumstances it may be desirable to form a conduit valve wherein flexible or rigid conduits are required to replace a missing portion of a patient""s aorta or to interface with an artificial blood pumping device. In such circumstances, an inlet conduit may be attached to the stent inflow and, if desired, a corresponding outflow conduit can be attached inside or outside of the valve wireform. Unlike prior art tissue heart valves, the present invention provides this flexibility and adaptability of use because key valve components can be standardized for different types of valves or valve applications. This manufacturing and structural consistency also improves quality control and provides repeatability and consistency in the formation of the valves. It also simplifies final assembly that in turn provides for increased production rates without sacrificing consistent product quality.
More specifically, as part of the flexibility of the present invention, the stent is designed to be adaptable so that different ways of attaching the valve to its various intended applications can be accommodated. The novel construction that allows for this universal application results from the stent providing a complete uniform support to the dimensionally stable, pre-aligned wireform/leaflet subassembly. Because of this adaptability, the valve of the present invention can function in a variety of applications, including that of a temporary heart valve prosthesis within a circulatory support system using a relatively rigid flange or a conduit assembly rather than a standard soft sewing ring. Alternatively, the present invention can function as a prosthetic valve having a soft, scallop-shaped sewing ring for aortic positioning or a soft flat sewing ring for mitral positioning, or as a conduit valve by incorporating proximal and distal conduits attached on both the inflow and outflow valve ends. The outflow conduit can have a sinus shape to improve blood flow if desired. Within an artificial heart system, the valve of the present invention mimics the hemodynamic pumping action of the heart while sustaining the patient until a donor heart is located and successfully transplanted. In this application, both blood inflow and outflow functions can be accommodated by the present invention.
Other objects and advantages of the present invention will become more apparent to those persons having ordinary skill in the art to which the present invention pertains from the following description taken in conjunction with the accompanying drawings.